On the source of non-linear light absorbance in photosynthetic samples

Abstract

This study presents a mathematical model, which expresses the absorbance of a photosynthetic sample as a non-linear polynomial of selected reference absorbance. The non-linearity is explained by inhomogeneities of a product of pigment concentration and light path length in the sample. The quadratic term of the polynomial reflects the extent of inhomogeneities, and the cubic term is related to deviation of the product distribution from a symmetric one. The model was tested by measurements of suspension of unstacked tobacco thylakoid membranes of different chlorophyll concentrations in cuvettes of different thicknesses. The absorbance was calculated from the diffuse transmittance and reflectance of sample, illuminated by perpendicular collimated light. The evaluated quantity was a sensitivity defined as the relative difference between the sample absorbance and the reference absorbance to the reference absorbance. The non-linearity of sample absorbance was demonstrated by a characteristic deviation of the sensitivity spectrum from a constant value. The absorbance non-linearity decreased on an increase of the product of pigment concentration and cuvette thickness. The model suggests that the sieve and detour effects influence the absorbance in a similar way. The model may be of interest in modeling of leaf or canopy optics including light absorption and scattering.

Light quality, oxygenic photosynthesis and more

“…Since Chl molecules are packed in chloroplasts and TMs inside chloroplasts, Chls that receive the light first, shield the others below them. This effect is known as the 'sieve effect' or 'flattening of absorption spectrum' since it leads to a decrease and broadening of absorption peaks of the pigments (Duysens 1956, Das et al 1967, Jávorfi et al 2006, Merzlyak et al 2009, Baránková et al 2016, Nauš et al 2018. On the other hand, there is 'scattering' of light (all optical effects causing changes in the direction of light propagation), which increases the path length of the excitation light beam in the sample, and thus, the probability of light absorption.…”

“…On the other hand, there is 'scattering' of light (all optical effects causing changes in the direction of light propagation), which increases the path length of the excitation light beam in the sample, and thus, the probability of light absorption. This effect is known as the detour effect (Kok 1948, Terashima and Saeki 1983, Merzlyak et al 2009, Baránková et al 2016, Nauš et al 2018. Hence, an increased concentration of the pigments causes a decrease of absorption due to the sieve effect, but also an increase of absorption due to the detour effect, and as a result, light absorbance is a nonlinear function (Baránková et al 2016, Nauš et al 2018.…”

“…This effect is known as the detour effect (Kok 1948, Terashima and Saeki 1983, Merzlyak et al 2009, Baránková et al 2016, Nauš et al 2018. Hence, an increased concentration of the pigments causes a decrease of absorption due to the sieve effect, but also an increase of absorption due to the detour effect, and as a result, light absorbance is a nonlinear function (Baránková et al 2016, Nauš et al 2018. Both the sieve and detour effects lead to similar relative changes in the absorption spectra of the photosynthetic sample: an increase in the green region, and a decrease in both the red and the blue region (Terashima et al 2009, Nauš et al 2018.…”